linux-kernel - Re: [PATCH] tools/memory-model: remove rb-dep, smp_read_barrier_depends, and lockless

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Message-Id: <20180220144813.GF3617@linux.vnet.ibm.com>
Date:   Tue, 20 Feb 2018 06:48:13 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        mingo@...nel.org, Will Deacon <will.deacon@....com>,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH] tools/memory-model: remove rb-dep,
 smp_read_barrier_depends, and lockless_dereference

On Mon, Feb 19, 2018 at 06:44:13PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 19, 2018 at 12:14:45PM -0500, Alan Stern wrote:
> > Note that operations like atomic_add_unless() already include memory 
> > barriers.
> 
> It is valid for atomic_add_unless() to not imply any barriers when the
> addition doesn't happen.

Agreed, given that atomic_add_unless() just returns 0 or 1, not the
pointer being added.  Of course, the __atomic_add_unless() function
that it calls is another story, as it does return the old value.  Sigh.
And __atomic_add_unless() is called directly from some code.  All of
which looks to be counters rather than pointers, thankfully.

So, do we want to rely on atomic_add_unless() always being
invoked on counters rather than pointers, or does it need an
smp_read_barrier_depends()?

							Thanx, Paul